Scattering bar OPC application method for sub-half wavelength lithography patterning

ABSTRACT

A method of forming a mask having optical proximity correction features, which includes the steps of obtaining a target pattern of features to be imaged, expanding the width of the features to be imaged, modifying the mask to include assist features which are placed adjacent the edges of the features to be imaged, where the assist features have a length corresponding to the expanded width of the features to be imaged, and returning the features to be imaged from the expanded width to a width corresponding to the target pattern.

CLAIM OF PRIORITY

This application is a Divisional of U.S. application Ser. No.10/880,376, filed Jun. 30, 2004 now U.S. Pat. No. 7,354,681, claimingpriority from U.S. provisional application No. 60/483,105, entitled “AMethod For Compensating For Scattering Bar Loss,” filed Jun. 30, 2003;and from U.S. provisional application No. 60/500,272, entitled “ImprovedScattering Bar OPC Application Method For Sub-Half WavelengthLithography Patterning,” filed Sep. 5, 2003, the entire contents of eachof which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to photolithography, and in particular toan improvement in optical proximity correction (OPC) by utilizing animproved scattering bar/assist feature design, as well as a new methodfor implementing scattering bars in a mask design.

BACKGROUND

Lithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, the mask may contain acircuit pattern corresponding to an individual layer of the IC, and thispattern can be imaged onto a target portion (e.g., comprising one ormore dies) on a substrate (silicon wafer) that has been coated with alayer of radiation-sensitive material (resist). In general, a singlewafer will contain a whole network of adjacent target portions that aresuccessively irradiated via the projection system, one at a time. In onetype of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionin one go; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus—commonly referred to as a step-and-scanapparatus—each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction; since, ingeneral, the projection system will have a magnification factor M(generally <1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Moreinformation with regard to lithographic devices as described herein canbe gleaned, for example, from U.S. Pat. No. 6,046,792, incorporatedherein by reference.

In a manufacturing process using a lithographic projection apparatus, amask pattern is imaged onto a substrate that is at least partiallycovered by a layer of radiation-sensitive material (resist). Prior tothis imaging step, the substrate may undergo various procedures, such aspriming, resist coating and a soft bake. After exposure, the substratemay be subjected to other procedures, such as a post-exposure bake(PEB), development, a hard bake and measurement/inspection of the imagedfeatures. This array of procedures is used as a basis to pattern anindividual layer of a device, e.g., an IC. Such a patterned layer maythen undergo various processes such as etching, ion-implantation(doping), metallization, oxidation, chemo-mechanical polishing, etc.,all intended to finish off an individual layer. If several layers arerequired, then the whole procedure, or a variant thereof, will have tobe repeated for each new layer. Eventually, an array of devices will bepresent on the substrate (wafer). These devices are then separated fromone another by a technique such as dicing or sawing, whence theindividual devices can be mounted on a carrier, connected to pins, etc.Further information regarding such processes can be obtained, forexample, from the book “Microchip Fabrication: A Practical Guide toSemiconductor Processing”, Third Edition, by Peter van Zant, McGraw HillPublishing Co., 1997, ISBN 0-07-067250-4, incorporated herein byreference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection systems,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Twin stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, incorporated herein by reference.

The photolithographic masks referred to above comprise geometricpatterns corresponding to the circuit components to be integrated onto asilicon wafer. The patterns used to create such masks are generatedutilizing CAD (computer-aided design) programs, this process often beingreferred to as EDA (electronic design automation). Most CAD programsfollow a set of predetermined design rules in order to create functionalmasks. These rules are set by processing and design limitations. Forexample, design rules define the space tolerance between circuit devices(such as gates, capacitors, etc.) or interconnect lines, so as to ensurethat the circuit devices or lines do not interact with one another in anundesirable way. The design rule limitations are typically referred toas “critical dimensions” (CD). A critical dimension of a circuit can bedefined as the smallest width of a line or the smallest space betweentwo lines. Thus, the CD determines the overall size and density of thedesigned circuit.

Of course, one of the goals in integrated circuit fabrication is tofaithfully reproduce the original circuit design on the wafer (via themask). Currently, various optical proximity correction (OPC) techniquesare utilized to allow the resulting image to more accurately correspondto the desired target pattern. A common OPC technique, which is widelyknown, is the use of subresolution scattering bars (also referred to asassist features). As described, for example, in U.S. Pat. No. 5,821,014,sub-resolution assist features, or scattering bars, are used as a meansto correct for optical proximity effects and have been shown to beeffective for increasing the overall process window (i.e., the range offocus and exposure dose variation over which features having somespecified CD can be printed consistently, regardless of whether or notthe features are isolated or densely packed relative to adjacentfeatures). As set forth in the '014 patent, generally speaking, theoptical proximity correction occurs by improving the depth of focus forthe less dense to isolated features by placing scattering bars nearthese features. The scattering bars function to change the effectivepattern density (of the isolated or less dense features) to be moredense, thereby negating the undesirable proximity effects associatedwith printing of isolated or less dense features. It is important,however, that the scattering bars themselves do not print on the wafer.Thus, this requires that the size of the scattering bars must bemaintained below the resolution capability of the imaging system.

Notwithstanding the wide-spread use of scattering bars, there remainsessentially three issues with current scattering bar technology whenutilized for patterning feature dimensions at half or below the exposurewavelength. The first issue relates to inadequate protection for themain design features that severely limits focus range. The second issuerelates to the fact that in a typical scattering bar solution, too manyshort pieces of scattering bars are generated which results in excessivedemands on mask making capabilities. The third issue relates to the factthat there is no adequate solution for adjacent horizontal and verticalscattering bars to be joined together. Current methods require that thehorizontal and vertical scattering bars be pulled apart from oneanother.

FIGS. 1 a-1 c illustrate the first issue noted above. FIG. 1 aillustrates an exemplary layout having both features to be printed 12and scattering bars 13 which perform OPC. FIGS. 1 b and 1 c illustratethe resulting printing performance at “best focus” and a defocus of 0.1um. As shown in FIG. 1 c, which has a portion of the resulting patternencircled which corresponds to the encircled portion of the mask of FIG.1 a, the areas 14 of features 12 which do not have any verticallypositioned scattering bars adjacent thereto exhibit “pinching” (i.e., aundesirable reduction in the width of the line to be printed).

FIG. 2 illustrates the second issue noted above. More specifically, FIG.2 illustrates a mask (also referred to herein as mask layout) modifiedto include scattering bars utilizing currently known techniques forscattering bar application. The mask includes both features 12 to beprinted and scattering bars 13. As shown in FIG. 2, current techniquesresult in an excessive number of short pieces of scattering bars 15 inthe mask layout. However, due to mask making process limitations, manyof these short pieces of scattering bars must be eliminated, therebyundesirably reducing printing performance.

FIG. 2 also illustrates the third issue noted above. As shown, none ofthe vertical and horizontal scattering bars 13 located proximate oneanother are connected to one another. This is due to the fact thatcurrent techniques for placing scattering bars within a mask designrequire that vertical and horizontal scattering bars, for example, whichare adjacent a corner of a feature to be printed, be pulled apart fromone another so as to prevent imaging of the intersecting portion of thescattering bars. However, as noted above, the elimination of scatteringbar portions from the mask results in an undesirable reduction inprinting performance.

Thus, there exists a need for a method of providing subresolutionscattering bars (also referred to as assist features) in a mask whichovercomes the foregoing problems so as to allow for improved OPC andprinting performance.

The following description discusses novel methods for applyingscattering bars to a mask layout.

SUMMARY

In an effort to solve the aforementioned needs, it is an object of thepresent invention to provide a method and technique for modifying a maskto include scattering bars, which decreases the amount of individualscattering bars included in the layout design, while increasing theoverall area occupied by the scattering bars. The method of the presentinvention also allows for the use of a novel “chamfer” scattering barwhich allows for the connection of adjacent vertical and horizontalscattering bars and thereby provides improved printing performance ofcorner features contained in the mask, as well as full protection (i.e.,completely surrounded by scattering bars) of isolated features.

More specifically, the present invention relates to a method ofmodifying a mask to include optical proximity correction features, whichincludes the steps of obtaining a target pattern of features to beimaged; expanding the width of the features to be imaged; modifying themask to include assist features which are placed adjacent the edges ofthe features to be imaged, where the assist features have a lengthcorresponding to the expanded width of the features to be imaged, andreturning the features to be imaged from the expanded width to a widthcorresponding to the target pattern. The resulting modified mask layoutcomprises the combination of the assist features and the features to beimaged having a width corresponding to the target pattern.

The present invention also relates to a method of forming a maskcomprising features to be imaged and optical proximity correctionfeatures. The method comprises the steps of forming a first assistfeature extending in a vertical direction; forming a second assistfeature extending in a horizontal direction; and forming a chamferassist feature which connects the first assist feature to the secondassist feature, where the chamfer assist feature is disposed at an anglerelative to both the first assist feature and the second assist feature.

The present invention provides numerous advantages over the prior arttechniques. One advantage is that the technique of the present inventiondecreases the amount of individual scattering bars included in the maskdesign, and increases the overall area occupied by the scattering bars.In addition, the method of the present invention provides for placementof scattering bars at line ends, which would be omitted utilizing priorart techniques. This results in a simplification of the mask makingprocess, while simultaneously improving printing performance. The methodalso allows for the use of a novel “chamfer” scattering bar which allowsfor the connection of adjacent vertical and horizontal scattering barsand thereby provides improved printing performance of corner featurescontained in the layout, as well as full protection (i.e., completelysurrounded by scattering bars) of isolated features. This completesurround assures that all parts of these design features receive thedepth of focus improvement that scattering bars are known to provide toisolated features.

The foregoing and other features, aspects, and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 c illustrate problems resulting from the application ofscattering bars utilizing currently known methods of applying scatteringbars to a mask design.

FIG. 2 illustrates an exemplary mask layout modified to includescattering bars utilizing currently known methods of applying scatteringbars to a mask layout.

FIG. 3 illustrates an exemplary mask layout modified to includescattering bars utilizing the method of the present invention.

FIG. 4 is an exemplary flow chart which sets forth the method ofapplying scattering bars to a mask layout in accordance with the presentinvention.

FIGS. 5 a-5 c illustrate the process described in the flowchart of FIG.4.

FIG. 6 illustrates the mask layout after completion of the scatteringbar application process in accordance with the present invention.

FIG. 7 illustrates the application of scattering bars to the same targetpattern shown in FIG. 6 utilizing prior art scattering bar techniques.

FIG. 8 is a flow chart of an exemplary method of generating the“chamfer” scattering bar in accordance with the present invention.

FIGS. 9-11 illustrate the process described in the flowchart of FIG. 8.

FIG. 12 a illustrates the use of the novel chamfer scattering bar of thepresent invention to connect two parallel scattering bar lines extendingin the same direction.

FIG. 12 b illustrates how the parallel scattering bars are connectedutilizing prior art techniques.

FIGS. 13 a and 13 b illustrate how the chamfer scattering bar of thepresent invention can be adjusted to optimize printing performance.

FIGS. 14 a and 14 b illustrate how the chamfers illustrated in FIGS. 13a and 13 b, respectively, can be utilized to cover outside corners of afeature.

FIGS. 15 a and 15 b illustrate how the chamfer scattering bars shown inFIGS. 13 a and 13 b, respectively, can be utilized in combination withhorizontal and vertical scattering bars to enclose a feature to beprinted.

FIG. 16 illustrates another embodiment of the chamfer scattering bar ofthe present invention.

FIG. 17 schematically depicts a lithographic projection apparatussuitable for use with a mask designed with the aid of the currentinvention.

DESCRIPTION

In accordance with the optical proximity correction technique of thepresent invention, there is provided a method and technique formodifying a mask layout to include scattering bars, which increases theamount of scattering bars included in the mask design, while minimizingthe number of individual pieces of scattering bars. The method alsoallows for the use of a novel “chamfer” scattering bar which allows forthe connection of adjacent vertical and horizontal scattering bars andthereby provides improved printing performance of corner featurescontained in the layout, as well as full protection (i.e., completelysurrounded by scattering bars) of isolated features.

FIG. 3 illustrates an exemplary mask layout modified to includescattering bars utilizing the method of the present invention, which isreferred to as the “scattering bar extension method”. Referring to. FIG.3, the mask layout includes horizontal features 31 and vertical features32, which are printed on the substrate, as well as horizontal scatteringbars 34 and vertical scattering bars 35. As noted above, the dimensionsof scatterings bars 34 and 35 are such that the scattering bars remainsubresolution, and do not print on the imaged substrate.

As shown, when utilizing the scattering bar extension method of thepresent invention, which will be described in detail below, theresulting mask layout includes significantly more scattering bars thancompared to prior art methods. This is clear from a comparison of FIG. 2and FIG. 3. It is noted that FIG. 3 illustrates the same underlyingpattern to be imaged as FIG. 2. Indeed, application of scatterings barsin accordance with the present invention results in the placement ofscattering bars in areas which would not contain scattering bars if thescattering bars were applied utilizing prior art methods. For example,referring to portions 36 of the mask layout, as shown in FIG. 3,horizontally positioned scattering bars 34 are positioned adjacent lineends of vertically positioned lines 32 utilizing the novel process.However, in contrast, when utilizing prior art methods, no suchhorizontal scattering bars would be placed adjacent the vertical lineends, as the width of the vertical line end would be deemed to small toowarrant placement of a scattering bar adjacent thereto.

FIG. 4 is an exemplary flow chart which sets forth the method ofapplying scattering bars to a mask layout in accordance with the presentinvention, and FIGS. 5 a-5 c illustrate the process of FIG. 4. Referringto FIG. 4, the first step 41 is to obtain the desired pattern (i.e.,target pattern) to be imaged on the substrate. As shown, the targetpattern contains both vertical features 32 and horizontal features 31(it is noted that to the extent features are referred to as vertical orhorizontal features herein it is for the sake of simplicity whenreferencing two sets of features which extend orthogonal to oneanother). The next step 42 is expand all features in the horizontaldirection, lengthening their horizontal edges. Any features sufficientlyclose together in this direction will then merge. The enlarged area ofthe horizontally-extended features is indicated by reference numeral 51in FIG. 5 a. The amount that the features are horizontally expanded instep 42 determines how far scattering bars will extend beyond the endsof horizontal edges of the target pattern, and at what maximumhorizontal separation features will be merged. In the given embodiment,this distance is selected to be approximately equal to the distancebetween a primary feature and its scattering bar (the distance to theclosest scattering bar, if multiple scattering bars are placed adjacentan edge). This criteria causes scattering bars at a convex corner toextend to a 45 degree angle outward. Beyond that, the resultinghorizontal scattering bar would be closer to a vertical edge than ahorizontal edge, where a vertical scattering bar would be more useful.It is also noted that, as shown in FIG. 5 a, the horizontally-extendedfeatures exhibit an extended horizontal edge surface. The next step 43is to generate horizontal scattering bars 57 adjacent each horizontaledge of the expanded features. In other words, the mask pattern ismodified to include horizontal scattering bars 57 placed adjacenthorizontal feature edges.

Once the horizontal scattering bars have been generated, the next stepin the process is to generate vertical scattering bars 59 to be placedadjacent the vertical feature edges in a similar manner. Referring againto FIG. 4, the first step 44 is to expand each feature in the verticaldirection. In the given embodiment, features sufficiently close in thevertical direction are expanded such that they join neighboringfeatures. The enlarged areas of the features are indicated by referencenumeral 56 in FIG. 5 b. It is noted that the amount of increase in thevertical dimension of the features is limited in the same manner as thehorizontal extension described above (i.e., in the given embodiment, theamount of the extension is chosen to be the distance between a primaryfeature and an adjacent scattering bar. It is also noted that, as shownin FIG. 5 b, the extended features exhibit an extended vertical edgesurface. The next step 45 is to generate vertical scattering bars 59adjacent each vertical edge of the vertically expanded features. Inother words, the mask pattern is modified to include vertical scatteringbars placed adjacent vertical feature edges.

It is further noted that in the given embodiment, when expanding thevertical and horizontal features, as shown in FIGS. 5 a and 5 b, eachvertical edge and horizontal edge is expanded in the respective steps(not simply features that would be deemed predominantly vertical orhorizontal). For example, referring to FIG. 5 b, the horizontal edge 61of the vertical feature 62 is extended during the expansion step. Thissubsequently results in the placement of an extended scattering baradjacent the vertical feature 61. All vertical edges are expanded in asimilar manner during step 42 as shown in FIG. 5 a, which results in theplacement of an extended scattering adjacent the horizontal feature 34.

Once step 45 is completed, in step 46, referring to FIG. 5 c, the masklayout having both extended vertical scattering bars and extendedhorizontal scattering bars are examined to determine any area in whichthe vertical scattering bars and the horizontal scattering bars interest(i.e., overlap) one another, and all of the intersecting areas arecancelled from the mask layout. In the final step of the process (step47), the expanded vertical and horizontal features are returned to theiroriginal size. Thus, after the foregoing process, the mask layoutcontains the original target pattern modified to include the extendedvertical and horizontal scattering bars. FIG. 5 c illustrates themodified mask layout prior to cancellation of the intersecting areas.

FIG. 6 illustrates the mask layout after completion of the foregoingscattering bar application process in accordance with the presentinvention. FIG. 7 illustrates the application of scattering bars to thesame target pattern shown in FIG. 6 utilizing prior art scattering bartechniques. As can be seen by a comparison of the drawings, the newscattering bar extension method results in scattering bar placement inareas not corrected utilizing prior art techniques. The new method alsoresults in a reduction in the number of individual scattering barsegments included in the mask design (i.e., many of the smaller,individual scattering bar designs are replaced with a single, continuousscattering bar). It is noted that the bottom portions of FIGS. 5 a-5 cand 6 do not represent the end of the mask pattern shown in thesefigures (i.e., only a portion of the entire target pattern isillustrated). If the bottom portion of these figures corresponded to theactual end of the target pattern, scattering bars would also be placedadjacent thereto. As explained further below, it also possible toconnect such adjacent vertical and horizontal scattering bars, which arecreated by the removal of the intersecting areas, utilizing a novel“chamfer” scattering bar as illustrated in FIGS. 13 a and 13 b.

The method of the present invention also entails utilizing novel“chamfer connection style” scattering bars, which allow for connectionbetween the ends of adjacent vertical and horizontal scattering bars(i.e., via the chamfer scattering bar). As discussed below, the chamferscattering bar eliminates the problems associated with utilizingscattering bars adjacent corner features of target patterns.

FIG. 8 is a flow chart describing an exemplary method of generating the“chamfer” scattering bar in accordance with the present invention.Referring to FIGS. 9-11, which illustrate the process, the first step 81is to place both a horizontal scattering bar 72 and a verticalscattering bar 73 adjacent a corner feature 74 of the target patternsuch that the horizontal and vertical scattering bars 72 and 73intersect with one another as shown in FIG. 9. In the next step (step83), the intersecting ends of the horizontal and vertical scatteringbars 72 and 73 are pulled back from one another such that two corners ofthe horizontal and vertical scattering bars contact one another as shownin FIG. 10. In the next step (Step 85), a triangular shaped feature 76(i.e., the chamfer scattering bar) is placed in contact with both theedge of the horizontal and vertical scattering bars 72 and 73 such thatthe face of the chamfer scattering bar 76 exposed to the corner featureforms a substantially 45° angle with the horizontal and verticalscattering bars 72 and 73 as shown in FIG. 1. It is noted that in thegiven embodiment, the chamfer scattering bar essentially forms atriangle feature having angles of 45°, 45° and 90°. It is further notedthat the chamfer scattering bar is substantially less likely to print ascompared to horizontal and vertical scattering bars which are allowed tointersect adjacent a corner region of a feature (which typically resultin the printing of an undesirable blob at the point of intersection).

It is also noted that while the chamfer scattering bar disclosed in thegiven embodiment exhibits a right triangle configuration have equalangles of 45°, it is not necessarily limited to such a configuration. Asdescribed below, the chamfer scattering bar is capable of otherconfigurations, each of which can be selected and optimized for a givenprocess and a given target pattern layout. One of the main aspects isthat the chamfer scattering bar exhibits a reduction in the area of thescattering bar in comparison to the square area that is formed/definedby the intersection of the vertical and horizontal scattering bars.

FIG. 12 a illustrates the use of the novel chamfer scattering bar toconnect two parallel scattering bar lines extending in the samedirection. In contrast, FIG. 12 b illustrates how the such parallelscattering have been connected utilizing prior art techniques. As shownin FIG. 12 a, the chamfer 91 is configured as a parallelogram havinglines which are approximately on a 45° angle relative to the verticallines of the scattering bars 92. In other words, in the givenembodiment, the chamfer 91 comprises two right triangles positionedside-to-side. Of course, the chamfer 91 can also be utilized to connectscattering bars disposed parallel to one another in the horizontaldirection. As shown below in FIG. 15, the use of the angled chamfer tocouple scattering bars extending in a parallel direction allows for theoverall scattering bar to more closely conform to the edge of thefeature to be printed. It is noted that the use of the chamfer of thepresent invention can be formed with more predictable results ascompared to the “corner touching” contacts shown in FIG. 12 b, andtherefore exhibits reduced issues associated with die-to-data baseinspection.

FIGS. 13 a and 13 b illustrate how the chamfer scattering bar of thepresent invention can be adjusted to optimize printing performance. FIG.13 a illustrates the same chamfer as disclosed above in FIG. 11, whereinthe chamfer 76 is formed of a right triangle having 45°, 45°, and 90°angles. FIG. 13 b illustrates a variation thereof in which the chamfer95 has a substantially trapezoidal configuration with both of theparallel lines are approximately on a 45° angle relative to the verticaland horizontal scattering bars.

It is noted that the use of the longer chamfer such as shown in FIG. 13b (as opposed to the chamfer shown in FIG. 13 a) may be preferred incertain situations as it may be easier to inspect on the mask due to thelonger length. For surrounding outside corners, assuming a circularlysymmetric printing system, the ideal scattering bar would be anarc-shaped scattering bar that maintains a constant distance from theprimary feature. This ideal scattering bar can be approximatedreasonably with 45-degree chamfers such as shown in FIG. 15 b.

While the chamfers shown in FIGS. 13 a and 13 b are utilized to improveimaging of inside corners of features in the target layout, it is alsopossible to utilize the chamfers to cover outside corners of features inthe layout. FIGS. 14 a and 14 b illustrate how the chamfers illustratedin FIGS. 13 a and 13 b, respectively, can be utilized to cover outsidecorners of a feature.

Further, as a result of the novel chamfer scattering bar, it is now alsopossible to fully enclose isolated features with scattering bars. FIGS.15 a and 15 b illustrate how the chamfer scattering bars shown in FIGS.13 a and 13 b can be utilized in combination with horizontal andvertical scattering bars to enclosed a feature to be printed. It is alsonoted that a combination of the chamfer scattering bars illustrated inFIGS. 13 a and 13 b can be utilized to enclose a given feature.

Yet another variation of the chamfer scattering bar is shown in FIG. 16.Referring to FIG. 16, the chamfer 97 is substantially arcuate (e.g., asemicircular configuration) and allows for line ends or corners ofvertical and horizontal scattering bars to be connected. As shown inFIG. 16, the chamfers 97 are utilized to couple two vertical scatteringbars 73 together so as to enclose the feature 74.

As noted above, the method and techniques of the present invention forforming scattering bars provides significant advantages over the priorart techniques. One advantage is that the technique of the presentinvention decreases the amount of individual scattering bars included inthe layout design, and increases the overall area occupied by thescattering bars. In addition, the method of the present inventionprovides for placement of scattering bars at line ends, which would beomitted utilizing prior art techniques. This results in a simplificationof the mask making process, while simultaneously improving printingperformance. The method also allows for the use of a novel “chamfer”scattering bar which allows for the connection of adjacent vertical andhorizontal scattering bars and thereby provides improved printingperformance of corner features contained in the layout, as well as fullprotection (i.e., completely surrounded by scattering bars) of isolatedfeatures, as well as maximum defocus protection for the features. Thiscomplete surround assures that all parts of these design featuresreceive the depth of focus improvement that scattering bars are known toprovide to isolated features.

It is noted that the method of the present invention can be implementedin software such that the foregoing methods disclosed herein areautomatically performed during the mask generation process. It isfurther noted that while not expressly described herein, the process ofidentifying the areas of intersections between the extended vertical andhorizontal scattering bars and the subsequent deletion thereof from themask design would be well known to those of skill in the art. As such, afurther description of that process is not provided herein. Further, itis noted that scattering bars are not placed adjacent every feature edgein the design. For example, features which are densely packed within thetarget design typically would not be candidates for placement ofscattering bars adjacent thereto.

FIG. 17 schematically depicts a lithographic projection apparatussuitable for use with a mask designed with the aid of the currentinvention. The apparatus comprises:

-   -   a radiation system Ex, IL, for supplying a projection beam PB of        radiation. In this particular case, the radiation system also        comprises a radiation source LA;    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g., a reticle), and connected to first        positioning means for accurately positioning the mask with        respect to item PL;    -   a second object table (substrate table) WT provided with a        substrate holder for holding a substrate W (e.g. a resist-coated        silicon wafer), and connected to second positioning means for        accurately positioning the substrate with respect to item PL;    -   a projection system (“lens”) PL (e.g., a refractive, catoptric        or catadioptric optical system) for imaging an irradiated        portion of the mask MA onto a target portion C (e.g., comprising        one or more dies) of the substrate W.

As depicted herein, the apparatus is of a transmissive type (i.e., has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (with a reflective mask). Alternatively, the apparatusmay employ another kind of patterning means as an alternative to the useof a mask; examples include a programmable mirror array or LCD matrix.

The source LA (e.g., a mercury lamp or excimer laser) produces a beam ofradiation. This beam is fed into an illumination system (illuminator)IL, either directly or after having traversed conditioning means, suchas a beam expander Ex, for example. The illuminator IL may compriseadjusting means AM for setting the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of theintensity distribution in the beam. In addition, it will generallycomprise various other components, such as an integrator IN and acondenser CO. In this way, the beam PB impinging on the mask MA has adesired uniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 17 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam that it produces being led into the apparatus (e.g., withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser (e.g.; based on KrF, ArFor F₂ lasing). The current invention encompasses at least both of thesescenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamPB, e.g., after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 17. However, in the case of a wafer stepper (as opposedto a step-and-scan tool) the mask table MT may just be connected to ashort stroke actuator, or may be fixed.

The depicted tool can be used in two different modes:

-   -   In step mode, the mask table MT is kept essentially stationary,        and an entire mask image is projected in one go (i.e. a single        “flash”) onto a target portion C. The substrate table WT is then        shifted in the x and/or y directions so that a different target        portion C can be irradiated by the beam PB;    -   In scan mode, essentially the same scenario applies, except that        a given target portion C is not exposed in a single “flash”.        Instead, the mask table MT is movable in a given direction (the        so-called “scan direction”, e.g., the y direction) with a speed        v, so that the projection beam PB is caused to scan over a mask        image; concurrently, the substrate table WT is simultaneously        moved in the same or opposite direction at a speed V=Mv, in        which M is the magnification of the lens PL (typically, M=¼ or        ⅕). In this manner, a relatively large target portion C can be        exposed, without having to compromise on resolution.

The concepts disclosed herein may simulate or mathematically model anygeneric imaging system for imaging sub wavelength features, and may beespecially useful with emerging imaging technologies capable ofproducing wavelengths of an increasingly smaller size. Emergingtechnologies already in use include EUV (extreme ultra violet)lithography that is capable of producing a 193 nm wavelength with theuse of a ArF laser, and even a 157 nm wavelength with the use of aFluorine laser. Moreover, EUV lithography is capable of producingwavelengths within a range of 20-5 nm by using a synchrotron or byhitting a material (either solid or a plasma) with high energy electronsin order to produce photons within this range. Because most materialsare absorptive within this range, illumination may be produced byreflective mirrors with a multi-stack of Molybdenum and Silicon. Themulti-stack mirror has a 40 layer pairs of Molybdenum and Silicon wherethe thickness of each layer is a quarter wavelength. Even smallerwavelengths may be produced with X-ray lithography. Typically, asynchrotron is used to produce an X-ray wavelength. Since most materialis absorptive at x-ray wavelengths, a thin piece of absorbing materialdefines where features would print (positive resist) or not print(negative resist).

While the concepts disclosed herein may be used for imaging on asubstrate such as a silicon wafer, it shall be understood that thedisclosed concepts may be used with any type of lithographic imagingsystems, e.g., those used for imaging on substrates other than siliconwafers.

Software functionalities of a computer system involve programming,including executable code, may be used to implement the above describedimaging model. The software code is executable by the general-purposecomputer. In operation, the code and possibly the associated datarecords are stored within a general-purpose computer platform. At othertimes, however, the software may be stored at other locations and/ortransported for loading into the appropriate general-purpose computersystems. Hence, the embodiments discussed above involve one or moresoftware products in the form of one or more modules of code carried byat least one machine-readable medium. Execution of such code by aprocessor of the computer system enables the platform to implement thecatalog and/or software downloading functions, in essentially the mannerperformed in the embodiments discussed and illustrated herein.

As used herein, terms such as computer or machine “readable medium”refer to any medium that participates in providing instructions to aprocessor for execution. Such a medium may take many forms, includingbut not limited to, non-volatile media, volatile media, and transmissionmedia. Non-volatile media include, for example, optical or magneticdisks, such as any of the storage devices in any computer(s) operatingas one of the server platform, discussed above. Volatile media includedynamic memory, such as main memory of such a computer platform.Physical transmission media include coaxial cables; copper wire andfiber optics, including the wires that comprise a bus within a computersystem. Carrier-Wave transmission media can take the form of electric orelectromagnetic signals, or acoustic or light waves such as thosegenerated during radio frequency (RF) and infrared (IR) datacommunications. Common forms of computer-readable media thereforeinclude, for example: a floppy disk, a flexible disk, hard disk,magnetic tape, any other magnetic medium, a CD-ROM, DVD, any otheroptical medium, less commonly used media such as punch cards, papertape, any other physical medium with patterns of holes, a RAM, a PROM,and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrierwave transporting data or instructions, cables or links transportingsuch a carrier wave, or any other medium from which a computer can readprogramming code and/or data. Many of these forms of computer readablemedia may be involved in carrying one or more sequences of one or moreinstructions to a processor for execution.

Although the present invention has been described and illustrated indetail, it is to be clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the scope of the present invention being limited only by theterms of the appended claims.

1. A method of forming a mask containing a target pattern comprisingfeatures to be imaged, said method comprising the steps of: (a)obtaining said target pattern comprising features to be imaged; (b)expanding the width of said features to be imaged; (c) modifying saidmask to include assist features, said assist features being placedadjacent edges of said features to be imaged, said assist featureshaving a length corresponding to the expanded width of said features tobe imaged; and (d) reducing the features to be imaged from the expandedwidth to a width corresponding to the target pattern; wherein said maskincludes said assist features and said features to be imaged, saidfeatures to be imaged having said width corresponding to said targetpattern.
 2. The method of forming a mask according to claim 1, whereinstep (b) and step (c) further comprise the steps of: expanding the widthof said features to be imaged in a first direction by a predeterminedamount; modifying said mask to include horizontally aligned assistfeatures which are disposed adjacent horizontal edges formed by saidexpanded features in a first direction; expanding the width of saidfeatures to be imaged in a second direction by a predetermined amount;and modifying said mask to include vertically aligned assist featureswhich are disposed adjacent vertical edges formed by said expandedfeatures in a second direction, said first direction and said seconddirection being orthogonal to one another.
 3. The method of forming amask according to claim 2, further comprising the step of canceling anyportion of said horizontally aligned assist features that overlap withsaid vertically aligned assist features from said mask, and cancelingany portion of said vertically aligned assist features that overlap withsaid horizontally aligned assist features from said mask.
 4. The methodof forming a mask according to claim 1, wherein said assist features aresubresolution such that said assist features do not print when said maskis imaged.
 5. The method of forming a mask according to claim 2, whereina single, continuous vertically aligned assist feature is disposedproximate line ends of horizontally aligned features that are disposedadjacent one another.
 6. The method of forming a mask according to claim2, wherein a single, continuous horizontally aligned assist feature isdisposed proximate line ends of vertically aligned features that aredisposed adjacent one another.
 7. A program product, comprisingexecutable code transportable by at least one machine readable medium,wherein execution of the code by at least one programmable computercauses the at least one programmable computer to perform a sequence ofsteps for forming a mask for optically transferring a pattern formed insaid mask onto a substrate, said steps comprising: (a) obtaining atarget pattern comprising features to be imaged; (b) expanding the widthof said features to be imaged; (c) modifying said mask to include assistfeatures, said assist features being placed adjacent edges of saidfeatures to be imaged, said assist features having a lengthcorresponding to the expanded width of said features to be imaged; and(d) reducing the features to be imaged from the expanded width to awidth corresponding to the target pattern; wherein said mask includessaid assist features and said features to be imaged, said features to beimaged having said width corresponding to said target pattern.
 8. Theprogram product according to claim 7, wherein step (b) and step (c)further comprise the steps of: expanding the width of said features tobe imaged in a first direction by a predetermined amount; modifying saidmask to include horizontally aligned assist features which are disposedadjacent horizontal edges formed by said expanded features in a firstdirection; expanding the width of said features to be imaged in a seconddirection by a predetermined amount; and modifying said mask to includevertically aligned assist features which are disposed adjacent verticaledges formed by said expanded features in a second direction, said firstdirection and said second direction being orthogonal to one another. 9.The program product according to claim 8, further comprising the step ofcanceling any portion of said horizontally aligned assist features thatoverlap with said vertically aligned assist features from said mask, andcanceling any portion of said vertically aligned assist features thatoverlap with said horizontally aligned assist features from said mask.10. The program product according to claim 7, wherein said assistfeatures are subresolution such that said assist features do not printwhen said mask is imaged.
 11. The program product according to claim 8,wherein a single, continuous vertically aligned assist feature isdisposed proximate line ends of horizontally aligned features that aredisposed adjacent one another.
 12. The program product according toclaim 8, wherein a single, continuous horizontally aligned assistfeature is disposed proximate line ends of vertically aligned featuresthat are disposed adjacent one another.